The present invention relates to a method of treating water in order to prevent the formation of calcium and magnesium salt deposits on the surfaces of pipes, boilers, evaporators, steam ejectors and the like as well as providing protection of metal surfaces against corrosion at ambient temperatures. The present invention also pertains to the prevention of barium sulphate scale formation in oil well drilling operations.
This prevention of scale and inhibition of corrosion are attained by adding to water a hydrolyzed terpolymer of maleic anhydride with vinyl acetate and a third monomer, preferably ethyl acrylate.
Various proposals have been made for the use of polymaleic anhydride and copolymers thereof in water treatment processes. For example, U.S. Pat. No. 3,617,577 describes the use of copolymers of maleic anhydride and ethylene with molecular weights of from about 1000 to 5000, and states that with polymers of lower molecular weight no substantial inhibition of scale depositon is achieved at threshold concentrations.
U.S. Pat. No. 2,723,956 describes the use of copolymers of maleic anhydride and another monoethylenic compound in a molar ratio of 1:2 to 2:1 for the reduction and prevention of scale formation, but the molecular weights are not disclosed. U.K. patent specification No. 772,775 describes a process for treating hard water with water soluble copolymers of maleic anhydride and other monomers, having molecular weights of from 1000 to 40000.
U.S. Pat. No. 3,558,570 discloses the preparation of telomers of styrene and maleic anhydride having a molecular weight under 2500.
U.S. Pat. No. 3,684,779 describes terpolymers of maleic monomers (such as maleic anhydride), acrylic monomers (such as acrylic acid) and alkenyl phosphonate monomers (such as diethyl vinylphosphonate) having a molecular weight between 5000 and 50000 which are sequestering or chelating agents useful in preventing scale formation.
U.S. Pat. No. 3,810,834 pertains to the use of hydrolyzed homopolymers of maleic anhydride as a water treatment to prevent scale formation or to modify said scale to facilitate its removal. The hydrolyzed homopolymer has a molecular weight between 300 to 5000. This product is particularly useful in preventing calcium carbonate and other alkali metal or alkaline earth metal carbonate scale.
U.S. Pat. No. 3,715,307 describes 1:1 mole ratio copolymers of maleic anhydride and selected vinyl monomers, particularly vinyl acetate. These copolymers are suitable as water conditioners to reduce maintenance problems associated with using sea water in heat transfer equipment.
U.S. Pat. No. 3,755,264 pertains to copolymers (and the hydrolyzed analogues thereof) of maleic anhydride (85 to 99 mol percent) with acrylic acid, vinyl acetate, styrene or mixtures thereof (15 to 1, preferably 15 to 5 mol percent) which are useful as scale inhibitors in water systems. The copolymers have a molecular weight of at least 300. While these copolymers are effective in preventing scale deposition of the carbonate type, they are relatively ineffective againt sulphate scale unlike the hydrolyzed terpolymers of the present invention which are very effective against sulphate.